The present invention relates to a process for detecting the speed of rotation of a DC electric motor of the type defined in the introduction to claim 1.
DC electric motors either with or without brushes, controlled with a pulse width modulated (PWM) square wave control signal are widely utilised, for example, on board motor vehicles for various applications such as driving the fan, moving windows and the like.
In such applications PWM controls signals of rather high frequency, for example in the order of 20 KHz are currently used.
In the case of a DC motor having brushes, it is known to detect the speed of rotation, for example for the purpose of regulation, by analysing the current consumed by the motor. This method of detecting the speed of rotation is however rather imprecise and highly influenced by ambient conditions (temperature, supply voltage variations, intrinsic tolerances of the motor etc). Other known arrangements for the detection of the speed of rotation of a DC electric motor involve the use of magnetic sensors (Hall effect sensors, or magnetoresitive sensors) or sensors of optical type. In all cases the adoption of sensors has a detrimental effect due to the inevitable structural complications and the necessary integration of the detector part with the control electronics of the motor, as well as the supplementary costs associated with the use of sensors.
Other known arrangements for detecting the speed of rotation without recourse to the use of sensors involve, in motors having brushes, the so called direct reading of the brushes, that is to say detection and analysis of the ripple in the current consumed during commutation. In the case of motors controlled with PWM signals having a frequency of the order of 10 KHz this technique is problematic to put into practice in that the signal represented by the ripple in the current has much more modest frequency and amplituded dynamics than the frequency and amplitude dynamics of the PWM control signals so that recourse to very sophisticated filters is necessary.
Another known method involves reading the electro motive force (EMF) or counter electromotive force (CEMF). In this case, however, the measurement is strongly influenced by the ambient conditions, and in particular by the temperature.